This invention relates to a metal-plastic multilayer pipe with a plastic inner pipe and metal sheath around the plastic inner pipe, suitable for use in hydronic heating systems and for domestic hot and cold water plumbing systems.
In hydronic heating systems good quality xe2x80x9ccross-linkedxe2x80x9d polyethylene tubing can carry water at temperature in excess of 140xc2x0 F. without any deterioration of the tubing or the tubing oxygen barrier. The design criteria of plastic tubing for radiant floor and wall hydronic heating systems is determined by a number of important factors to insure an absolutely safe and reliable tubing. The same design criteria are often demanded for hot and cold water domestic plumbing systems. The most important design criteria requirements are:
1. High resistance to temperature aging for water temperatures up to 200xc2x0 F.
2. High resistance to stress cracking.
3. High resistance to chemical solvents (water additives, antifreeze solutions, concrete additives).
4. Lowest possible linear thermal expansion.
5. High tensile strength.
6. High form stability.
7. High resistance to abrasion.
8. High resistance to deformation.
9. Dimensional tube tolerances.
9. Internal and external tube wall smoothness.
10. Behavior during long term internal pressure creep test which takes into account the temperature-dependent aging behavior of the pipe material at water temperatures up to 200xc2x0 F.
Many of these requirements are dictated by the usual practice of embedding the tubing in a layer of concrete. They are design criteria that are outlined and specified in the ASTM standards (American Society for Testing and Materials), and DIN (German Industry Standards). Many, if not all of these design requirements can be achieved while still retaining a flexible and workable plastic tubing (pipe) as an end product. That tubing is called PEX, which is short for xe2x80x9cPolyethelene Cross-Linkedxe2x80x9d. PEX has been synonymous with plastic heating pipe in many European countries for several decades and has a track record that has made it the plastic tubing of choice for hydronic heating applications. Long term bench tests, which simulate 30 years of continuous use, in addition to accelerated testing which projects pipe performance well in excess of 30 years has confirmed the excellent long term real service life track record of PEX. Crossed-linked polyethylene tubing is now, after 20 years of use and improvements, the most widely accepted pipe material in the European plumbing industry for both heating and plumbing applications.
The molecules of any plastic material tend to slip and slide over one another fairly freely. As ambient and water temperatures rise, the plastic material softens and finally melts. This thermal oxidation of plastic material is a long term aging process which will eventually result in pipe failure.
To combat this premature aging the molecules within the tubing are realigned in order to give greater stability to the material itself. The cross linking process takes place within the molecular structure of the plastic material. The most common thermoplastic materials currently being used for heating and plumbing pipe, often referred to as polyolefin materials are: Polyethylene (PE); Polypropylene (PP); and Polybuten (PB)(generic term for polybutylene). Among this family of polyolefin plastics, only Polyethylene has been determined to have the molecular structure which lends itself perfectly to the cross linking process.
xe2x80x9cUn-cross-linkedxe2x80x9d polyethylene (PE) tubing, as it leaves the extruder where it receives its basic pipe dimension and wall thickness, is composed of long hydrocarbon string molecules forming a loosely held together array of hydrogen and carbon atoms which can be compared to a beaded curtain swaying in a breeze. This is basically the molecular composition of the PE tubing which is available at hardware stores and is suitable only for non-critical applications. A material, in this form, is not suitable for heating and plumbing applications. Within a relatively short period of time the pipe material fatigues under the stress of water temperature and pressure as well as temperature cycling and the beaded curtain of molecules splits open without resistance. By cross-linking the molecular xe2x80x9cbeadsxe2x80x9d (hydrocarbon string molecules), forming cross-connections which are referred to as cross-linking bridges, the string molecules form a three dimensional network of hydrocarbon molecules. The xe2x80x9cbeaded curtainxe2x80x9d becomes transformed into a fishing net with strength and stability.
In this way, the previously non-applicable PE pipe is transformed, after cross-linking, into a completely different material with all the desired characteristics demanded for a heating or plumbing pipe. After the crosslinking of the PE tubing, its molecular mobility is severely impeded by the cross-linking bridges between the string molecules. The material does not flow or melt and its form becomes stable against heat. The material holds its shape at all temperatures, even exposed to blow torch temperatures until it chars or burns. The thermoplastic has been transformed into a thermoset material by cross-linking, eliminating the melting point or liquid phase of the material. The PE pipe has been transformed into PEX pipe
There are basically two types of PE raw materials in use: Low to Medium density (LD or MD PE) and High density (HD PE). Low to Medium density polyethylene xe2x80x9cSOFT PExe2x80x9d has a multibranch string molecule shape which allows a lower to medium density formation of string molecules within the pipe PE material.
High density polyethylene (HD PE) has a linear string molecule shape with only small stumps of branches, which allows for a higher density formation of string molecules within the pipe PE material.
The material density affects the physical properties of the pipe material. HD PE or xe2x80x9cHARD PExe2x80x9d has a higher resistance to stress cracking and chemical solvents, higher tensile strength, higher resistance to deformation and is less permeable to oxygen
Chemical Cross-linking techniques include: Peroxide Cross-linking; Silan Cross-linking via Dow Corning Method; and AZO Cross-Linking. Three methods of Peroxide Cross-Linking are the Engel, PAM and DAOPLAST methods.
The various chemical cross-linking methods use chemical agents which are added to the PE base resin in order to form cross-linking bridges between the PE string molecules. The only practiced mechanical cross-linking method uses no chemical agents, instead, utilizes the high energy of an electron beam accelerator to form a three dimensional cross-linking network between the PE molecules.
Among the various chemical methods only two types are commonly used for heating pipe production: The Engel and Silan method. The Engel method, named after its inventor, uses a cross linking agent (peroxide) and heat stabilization agents which are mixed into the PE resin. The mixture is then compressed under high pressure in a xe2x80x9cpre-moltenxe2x80x9d state and fed through the extrusion die, where the actual cross linking process takes place. This is a xe2x80x9cpress-sinteringxe2x80x9d process which achieves pipe extrusion and molecular cross-linking during one extrusion process.
The Silan method uses a mixture of two compounds with a mixing ratio of 95 to 5 parts. One compound consists of PE resin and cross-linking agents as well as other additives. The second compound consists of PE resin and a catalyst. After mixing both compounds, the pipe is extruded conventionally. The cross-linking reaction is triggered after extrusion by exposing the extruded coil to moisture such as steam or water. Most other chemical methods are variations of either the Engel or Silan method.
The electronic or mechanical cross linking method does not use any chemical means to achieve cross-linking bridges between the PE molecules. The basic PE resin is first extruded to give the pipe the basic required dimensional shape, then coiled up and fed through a high energy electron accelerator which exposes the extruded pipe material to the enormous energy of an electron beam. The energetic electrons strike the PE molecules at or near a carbon/hydrogen bond, releasing enough energy to the molecule to break that bond, setting the hydrogen atom free which diffuses out of the pipe in the form of hydrogen gas during the process. A large percentage of carbon atoms have then lost their hydrogen atom partner leaving the parent molecule in an excited state, able to form a new bond with another adjacent carbon atom without a hydrogen partner (called a free radical). These new carbon to carbon bonds are the desired cross linking bridges which form a three dimensional network among the PE string molecules.
Electronic cross-linking is the oldest known method of creating PEX. Over three decade ago, U.S. Companies applied this method to manufacture primarily cable enclosures including those used for undersea cables. This method was then adopted by Hewing GmbH of Germany for the production of heating and plumbing pipes.
A closed loop hydronic heating system will cause an oxygen poor water condition after the initial filling of the piping system. Oxygen depleted water (dead water) in an oxygen tight closed piping system is an effective and inexpensive heat transfer medium. The benefit of oxygen depleted water is its non-corrosiveness to system components, piping, valves, pumps, boilers, etc. However, this generated xe2x80x9coxygen vacuumxe2x80x9d within a closed piping system causes a very high affinity in the system water for oxygen enrichment. This means that the generated oxygen vacuum in the system will absorb outside oxygen through any means possible.
In a steel or copper piping system the only source of oxygen permeation is through leaking fittings, valves, air vents, and above all, improperly sized expansion tank equipment. Copper or steel tubing itself is absolutely oxygen tight. However, that is not the case with plastic or rubber tubing. In recent years it has been discovered in Europe, after enormous corrosion and subsequent sludging problems developed in systems utilizing oxygen permeable plastic tubing in xe2x80x9cclosed systemsxe2x80x9d, that plastic tubing allowed enough oxygen permeation through the pipe wall to cause corrosion in the system.
Subsequently, a special oxygen diffusion test for plastic tubing was developed to determine the amount of oxygen penetrating the tubing. The chemically bound oxygen (no visible air bubbles) in the system water entering through the pipe walls creates an extremely aggressive water condition, corroding not only ferrous materials but also copper, brass and plastics as well. The tests revealed that the rate of oxygen diffusion is directly related to the system water temperaturexe2x80x94the higher the water temperature, the higher the rate of diffusion which is measured in milligrams per liter per day.
The German Industry standards (DIN) have determined that an oxygen diffusion rate of 0.1 mg/liter/day or less at a water temperature of 104xc2x0 F. (40xc2x0 C.) in plastic tubing is considered a safe level to prevent oxygen corrosion in heating system components. For comparison: The amount of 5 milligrams of oxygen per liter per day caused by oxygen diffusion through the pipe wall is equivalent to completely draining the heating system and refilling it with fresh water every other day during the heating season.
In order to eliminate the serious problems of oxygen diffusion on closed loop heating systems with plastic pipe, oxygen diffusion barriers have been developed. These barriers are usually applied to the exterior of the pipe. Each pipe manufacturer has its own method and process for applying this barrier. The main criteria for these barrier application techniques are the operating water temperatures of the intended pipe usage.
Oxygen diffusion is obviously no issue for plastic tubing intended for use on open hot and cold domestic hot water systems (plumbing systems) where oxygen is present at high concentrations in any case. However the aggressive corroding condition is prevented by the continuous dilution of fresh water to the system.
Heretofore, metal-plastic multilayer pipes have been manufactured continuously by placing a metal band around an extruded inner PE pipe. The metal band in its lengthwise extension is fed parallel to the axial extension of the inner PE pipe and laid around the inner pipe through forming. In the process, the longitudinal borders of the metal band join and are welded together (longitudinal joint welder) or the longitudinal borders overlap and are bonded in the overlapping area. The bonding agents used for this purpose are fairly sensitive to temperature, therefore there is a risk of leaks occurring in the bonded joint, and thus the function of the metal sheath of the inner pipe as diffusion and vapor barrier is no longer ensured.
This problem has been overcome by Hewing GmbH of Ochtrup, Germany, who use a bonding agent which creates an inseparable joint of the overlapping longitudinal borders of the metal band with a view to the expected temperature and mechanical load. As a consequence, the bonded joint does not consist of thermoplastic, but rather of duroplastic synthetic material. A two component or mixed adhesive, preferably on epoxy resin basis is especially suited for this purpose. In particular, the inner pipe is a PEX pipe and on the outside around the metal sheath, there is a practical (protective) plastic outer pipe which is extruded onto the metal sheath.
These metal-plastic multilayer pipes manufactured by Hewing GmbH satisfy the criteria for hydronic heating and have an excellent oxygen diffusion barrier that is required for closed hydronic heating systems and they satisfy all requirements of domestic hot and cold water plumbing systems. However, for both systems, and particularly for plumbing, installation throughout a building requires connecting many fittings, often one at every turn of the pipe and often at difficult to reach places.
Furthermore, in the past, the metal-plastic multilayer pipes for plumbing and heating have met SDR requirements specified in the ASTM standards, and DIN only by making the metal sheath a major structural part of the pipe and so the inner plastic pipe (which may be PEX) was not strong enough by itself to meet those requirements. As a consequence, the metal sheath accounted for a substantial part of the overall strength and stiffness of the multilayer pipe and could be bent by hand only with difficulty.
It is the principal object of the present invention to provide pipes that satisfy the criteria for hydronic heating and have an excellent oxygen diffusion barrier that is required for closed hydronic heating systems and also satisfy all requirements of hot and cold domestic hot water plumbing systems and have xe2x80x9cform stabilityxe2x80x9d so that the pipe can be bent readily into any shape without kinking and will retain the shape it is bent to.
According to an embodiment of the present invention a metal-plastic multilayer pipe is provided with a plastic inner pipe and a metal sheath around the plastic inner pipe, where the metal sheath is of uniform thickness is applied to the plastic inner pipe so that it completely encircles the plastic inner pipe, with the feature that the ratio of the thickness of the metal sheath to the total thickness of all layers of the pipe is substantially less than in the past and so the metal sheath accounts for a negligible part of the overall strength and stiffness of the multilayer pipe and is such that the pipe can be readily bent by hand and the metal layer offers negligible resistance to such bending as compared to the resistance offered by the inner pipe and any practical outside cover layer, so that the pipe has xe2x80x9cform stabilityxe2x80x9d. This means that the pipe can be bent readily into any shape without kinking and will retain the shape it is bent to.
Furthermore, the metal layer even though relatively very thin is a complete oxygen barrier. Thus the inner pipe may be PEX to satisfy all of the the criteria for hydronic heating and plumbing and the metal layer provides substantially only an oxygen diffusion barrier that is required for closed hydronic heating systems and the xe2x80x9cform stabilityxe2x80x9d.